Ac motor control method and control device

ABSTRACT

It is possible to control a direct current intermediate voltage to be constant, without using a regenerative resistor and an apparatus for feeding back regenerative energy to a power supply, and thus to stably and continuously drive an alternating current motor even at the time of a power failure. A method of controlling the alternating current motor includes the steps of: allowing a power failure detecting unit provided in a power converter to detect a power failure of an alternating current power source; outputting a deceleration start instruction to the inverter unit; calculating a first reduction rate, on the basis of a detection value and a target value of the direct current intermediate voltage, such that the direct current intermediate voltage is made constant during the deceleration of the alternating current motor; calculating a second reduction rate on the basis of a variation in the direct current intermediate voltage; calculating a torque instruction to allow the alternating current motor to be decelerated for a deceleration time; changing an electromotive torque limit value and a regenerative torque limit value on the basis of the value of the detected direct current intermediate voltage; stopping the deceleration when the direct current intermediate voltage is equal to a voltage before the power failure is detected or it rises during the deceleration; and when the alternating current motor is returned to a normal control mode, storing an output frequency before the power failure is detected.

TECHNICAL FIELD

The present invention relates to a method of controlling an alternatingcurrent motor and a control device therefore capable of continuouslydriving the alternating current motor at the time of an instantaneouspower failure.

BACKGROUND ART

As an example of a conventional method of continuously driving analternating current motor by using an inverter at the time of aninstantaneous power failure, Japanese Patent No. 3201460 (PatentDocument 1) discloses the following method: an inverter startsdecelerating an alternating motor in response to a power failuredetecting signal; a first reduction rate is calculated on the basis of atarget value and a detection value of a direct current intermediatevoltage such that the direct current intermediate voltage is madeconstant during the deceleration; a second reduction rate is calculatedon the basis of a variation in the direct current intermediate voltage;a value obtained by multiplying the two reduction rates together iscontrolled in a PI control manner to control a deceleration time; andwhen the direct current intermediate voltage is equal to a voltagebefore the power failure is detected or the direct current intermediatevoltage rises during the deceleration, the deceleration stops and thealternating current motor is returned to a normal control mode.

Further, as an example of a method of processing a motor at the time ofa power failure, JP-A-11-208894 (Patent Document 2) discloses thefollowing method: a lower limit voltage V_(U1) of a direct currentintermediate voltage required for a normal driving mode, a lowerallowable voltage V_(U0) of the direct current intermediate voltage whena lowest voltage of a power supply is input, and a power failuredetecting level voltage V_(U2) which is lower than the lower allowablevoltage V_(U0) and is higher than the lower limit voltage V_(U1) areset; when a detection value V_(PN) of the direct current intermediatevoltage output from a direct current intermediate voltage detectingcircuit is lower than the power failure detecting level voltage V_(U2)during the driving of the alternating current motor, an alternatingcurrent motor is decelerated at a reduction rate α_(d) until the directcurrent intermediate voltage is higher than the lower allowable voltageV_(U0); and when the direct current intermediate voltage is higher thanthe lower allowable voltage V_(U0), the alternating current motor isreturned to a normal control mode.

Patent Document 1: Japanese Patent No. 3201460

Patent Document 2: JP-A-11-308894

Problems to be Solved by the Invention

In the method of continuously driving the alternating current motor byusing the inverter at the time of a power failure disclosed in theabove-mentioned Patent Document 1, the first reduction rate iscalculated on the basis of the target value and the detection value ofthe direct current intermediate voltage, and the second reduction rateis calculated on the basis of the variation in the direct currentintermediate voltage. Then, the value obtained by multiplying the tworeduction rates together is controlled in the PI control manner tocontrol the deceleration time. When the direct current intermediatevoltage is equal to the voltage before the power failure is detected orthe direct current intermediate voltage rises during the deceleration,the deceleration stops, and the alternating current motor is returned tothe normal control mode.

However, in this method, when the alternating current motor is returnedto the normal control mode, a large difference occurs between an outputfrequency before the power failure and an output frequency during thedeceleration due to the power failure, causing the alternating currentmotor to be rapidly accelerated so as to coincide with an outputfrequency instruction or a large torque instruction to be outputted,which makes it difficult to stably and continuously drive thealternating current motor.

Meanwhile, in the method of processing the motor at the time of a powerfailure disclosed in Patent Document 2, when the detection value V_(PN)of the direct current intermediate voltage is lower than the powerfailure detecting level voltage V_(U2), the alternating current motor isdecelerated at the reduction rate α_(d) until the direct currentintermediate voltage is higher than the lower allowable voltage V_(U0).In addition, when the direct current intermediate voltage is higher thanthe lower allowable voltage V_(U0), the alternating current motor isreturned to the normal control mode. When the alternating current motoris returned to the normal control mode, a large difference occursbetween an output frequency before the power failure and an outputfrequency during the deceleration due to the power failure, causing thealternating current motor to be rapidly accelerated so as to coincidewith an output frequency instruction or a large torque instruction to beoutputted, which makes it difficult to stably and continuously drive thealternating current motor.

Accordingly, the invention is made in view of the above-mentionedproblems, and it is an object of the invention to a method ofcontrolling an alternating current motor and a control device thereforcapable of stably and continuously driving the alternating current motorat the time of an instantaneous power failure, by storing an outputfrequency when the instantaneous power failure occurs and byaccelerating the alternating current motor for an acceleration timeseparately set at a direct current intermediate voltage where thealternating current motor can return to a normal control mode until theoutput frequency of the alternating current motor is equal to an outputfrequency before the instantaneous power failure, or by restricting atorque instruction with an electromotive torque limit value until theoutput frequency of the alternating current motor is equal to the outputfrequency before the instantaneous power failure.

Means for Solving the Problems

In order to achieve the above-mentioned object, according to an aspectof the invention disclosed in claim 1, there is provided a method ofcontrolling an alternating current motor and a power converter includinga converter unit for converting an alternating current voltage suppliedfrom an alternating current power supply into a direct current voltage,a smoothing capacitor for smoothing the converted direct currentvoltage, and an inverter unit for converting a direct currentintermediate voltage into an alternating current voltage having afrequency corresponding to a torque instruction by a PWM control manner,by setting an electromotive torque limit value and a regenerative torquelimit value in advance and by restricting the torque instruction withthe torque limit values to generate a PWM switching pattern to be outputto the inverter unit. The method includes the steps of: allowing a powerfailure detecting unit provided in the power converter to detect a powerfailure of the alternating current power source; outputting adeceleration start instruction to the inverter unit in response to apower failure detecting signal output from the power failure detectingunit; calculating a first reduction rate, on the basis of a detectionvalue and a target value of the direct current intermediate voltage,such that the direct current intermediate voltage is made constantduring the deceleration of the alternating current motor; calculating asecond reduction rate on the basis of a variation in the direct currentintermediate voltage; controlling a deceleration time by controlling avalue obtained by multiplying the two reduction rates together in a PIcontrol manner; calculating the torque instruction to allow thealternating current motor to be decelerated for the deceleration time;changing the electromotive torque limit value and the regenerativetorque limit value on the basis of the value of the detected directcurrent intermediate voltage; stopping the deceleration when the directcurrent intermediate voltage is equal to a voltage before the powerfailure is detected or it rises during the deceleration; and when thealternating current motor is returned to a normal control mode, storingan output frequency before the power failure is detected.

According to the method of controlling an alternating current motordescribed in claim 2, preferably, when the direct current intermediatevoltage is equal to the voltage before the power failure is detected orit rises during the deceleration, the deceleration stops. In addition,preferably, when the alternating current motor is returned to the normalcontrol mode, the alternating motor is accelerated for an arbitraryacceleration time until its output frequency is equal to the outputfrequency stored before the power failure is detected.

According to the method of controlling an alternating current motordescribed in claim 3, preferably, when the direct current intermediatevoltage is equal to the voltage before the power failure is detected orit rises during the deceleration, the deceleration stops. In addition,preferably, when the alternating current motor is returned to the normalcontrol mode, a torque limit unit restricts the torque instruction withan arbitrary electromotive torque limit value until the output frequencyof the alternating current motor is equal to the output frequency storedbefore the power failure is detected.

According to another aspect of the invention described in claim 4, thereis provided a control device which controls an alternating current motorand a power converter including a converter unit for converting analternating current voltage supplied from an alternating current powersupply into a direct current voltage, a smoothing capacitor forsmoothing the converted direct current voltage, and an inverter unit forconverting a direct current intermediate voltage into an alternatingcurrent voltage having a frequency corresponding to a torque instructionby a PWM control manner, by setting an electromotive torque limit valueand a regenerative torque limit value in advance and by restricting thetorque instruction with the torque limit values to generate a PWMswitching pattern to be output to the inverter unit. The control deviceincludes: a power failure detecting unit which detects a power failureof the alternating current power source; a unit which outputs adeceleration start instruction to the inverter unit in response to apower failure detecting signal output from the power failure detectingunit, calculates a first reduction rate, on the basis of a detectionvalue and a target value of the direct current intermediate voltage,such that the direct current intermediate voltage is made constantduring the deceleration of the alternating current motor, and calculatesa second reduction rate on the basis of a variation in the directcurrent intermediate voltage; a unit which controls a deceleration timeby controlling a value obtained by multiplying the two reduction ratestogether in a PI control manner; a unit which calculates the torqueinstruction to allow the alternating current motor to be decelerated forthe deceleration time and changes the electromotive torque limit valueand the regenerative torque limit value on the basis of the value of thedetected direct current intermediate voltage; a unit which stopsdeceleration when the direct current intermediate voltage is equal to avoltage before the power failure is detected or it rises during thedeceleration; and a unit which stores an output frequency before thepower failure is detected when the alternating current motor is returnedto a normal control mode.

According to the control device for an alternating current motordescribed in claim 5, preferably, when the direct current intermediatevoltage is equal to the voltage before the power failure is detected orit rises during the deceleration, the deceleration stops. In addition,preferably, when the alternating current motor is returned to the normalcontrol mode, the alternating motor is accelerated for an arbitraryacceleration time until its output frequency is equal to the outputfrequency stored before the power failure is detected.

According to the control device for an alternating current motordescribed in claim 6, preferably, when the direct current intermediatevoltage is equal to the voltage before the power failure is detected orit rises during the deceleration, the deceleration stops. In addition,preferably, when the alternating current motor is returned to the normalcontrol mode, the torque limit unit restricts the torque instructionwith an arbitrary electromotive torque limit value until the outputfrequency of the alternating current motor is equal to the outputfrequency stored before the power failure is detected.

According to yet another aspect of the invention described in claim 7,there is provided a method of controlling an alternating current motorand a power converter including a converter unit for converting analternating current voltage supplied from an alternating current powersupply into a direct current voltage, a smoothing capacitor forsmoothing the converted direct current voltage, and an inverter unit forconverting a direct current intermediate voltage into an alternatingcurrent voltage having a frequency corresponding to a torque instructionby a PWM control manner. The method includes the steps of: setting anelectromotive torque limit value and a regenerative torque limit valuein advance; restricting the torque instruction with the torque limitvalues to generate a PWM switching pattern to be output to the inverterunit; setting a lower limit voltage V_(U1) of the direct currentintermediate voltage required for a normal driving mode, a lowerallowable voltage V_(U0) of the direct current intermediate voltage whena lowest voltage of a power supply is input, and a power failuredetecting level voltage V_(U2) which is lower than the lower allowablevoltage V_(U0) and is higher than the lower limit voltage V_(U1); when adetection value of the direct current intermediate voltage output from adirect current intermediate voltage detecting circuit is lower than thepower failure detecting level voltage V_(U2) during the driving of thealternating current motor, decelerating the alternating current motor ata reduction rate α_(d) until the direct current intermediate voltage ishigher than the lower allowable voltage V_(U0); when the direct currentintermediate voltage is higher than the lower allowable voltage V_(U0),returning the alternating current motor to a normal control mode; andwhen the direct current intermediate voltage is higher than the lowerallowable voltage V_(U0) to cause the alternating current motor to bereturned to the normal control mode, storing an output frequency beforethe power failure is detected.

According to the method of controlling an alternating current motordescribed in claim 8, preferably, when the direct current intermediatevoltage is higher than the lower allowable voltage V_(U0) to cause thealternating current motor to be returned to the normal control mode, thealternating current motor is accelerated for an arbitrary accelerationtime until its output frequency is equal to the output frequency storedbefore the power failure is detected.

According to the method of controlling an alternating current motordescribed in claim 9, preferably, when the direct current intermediatevoltage is higher than the lower allowable voltage V_(U0) to cause thealternating current motor to be returned to the normal control mode, atorque limit unit restricts the torque instruction with an arbitraryelectromotive torque limit value until the output frequency of thealternating current motor is equal to the output frequency stored beforethe power failure is detected.

According to still another aspect of the invention described in claim10, there is provided a control device which controls an alternatingcurrent motor and a power converter including a converter unit forconverting an alternating current voltage supplied from an alternatingcurrent power supply into a direct current voltage, a smoothingcapacitor for smoothing the converted direct current voltage, and aninverter unit for converting a direct current intermediate voltage intoan alternating current voltage having a frequency corresponding to atorque instruction by a PWM control manner. The control device controlsthe alternating current motor in the following sequence of: setting anelectromotive torque limit value and a regenerative torque limit valuein advance; restricting the torque instruction with the torque limitvalues to generate a PWM switching pattern to be output to the inverterunit; setting a lower limit voltage V_(U1) of the direct currentintermediate voltage required for a normal driving mode, a lowerallowable voltage V_(U0) of the direct current intermediate voltage whena lowest voltage of a power supply is input, and a power failuredetecting level voltage V_(U2) which is lower than the lower allowablevoltage V_(U0) and is higher than the lower limit voltage V_(U1); when adetection value of the direct current intermediate voltage output from adirect current intermediate voltage detecting circuit is lower than thepower failure detecting level voltage V_(U2) during the driving of thealternating current motor, decelerating the alternating current motor ata reduction rate α_(d) until the direct current intermediate voltage ishigher than the lower allowable voltage V_(U0); when the direct currentintermediate voltage is higher than the lower allowable voltage V_(U0),returning the alternating current motor to a normal control mode; andwhen the direct current intermediate voltage is higher than the lowerallowable voltage V_(U0) to cause the alternating current motor to bereturned to the normal control mode, storing an output frequency beforethe power failure is detected.

According to the control device for an alternating current motordescribed in claim 11, preferably, when the direct current intermediatevoltage is higher than the lower allowable voltage V_(U0) to cause thealternating current motor to be returned to the normal control mode, thealternating current motor is accelerated for an arbitrary accelerationtime until its output frequency is equal to the output frequency storedbefore the power failure is detected.

According to the control device for an alternating current motordescribed in claim 12, preferably, when the direct current intermediatevoltage is higher than the lower allowable voltage V_(U0) to cause thealternating current motor to be returned to the normal control mode, atorque limit unit restricts the torque instruction with an arbitraryelectromotive torque limit value until the output frequency of thealternating current motor is equal to the output frequency stored beforethe power failure is detected.

According to still yet another aspect of the invention described inclaim 13, a method of controlling an alternating current motor includesthe steps of: providing a power failure detecting unit for detecting apower failure of the alternating current motor in a power converter;outputting a deceleration start instruction to an inverter unit inresponse to a power failure detecting signal output from the powerfailure detecting unit; calculating a first reduction rate, on the basisof a detection value and a target value of a direct current intermediatevoltage, such that a direct current intermediate voltage is madeconstant during the deceleration of the alternating current motor;calculating a second reduction rate on the basis of a variation in thedirect current intermediate voltage; controlling a deceleration time bycontrolling a value obtained by multiplying the two reduction ratestogether in a PI control manner; when the direct current intermediatevoltage is equal to a voltage before the power failure is detected or itrises during the deceleration, stopping the deceleration and returningthe alternating current motor to a normal control mode; and when thedirect current intermediate voltage is equal to the voltage before thepower failure is detected or it rises during the deceleration to causethe deceleration to stop and the alternating current motor to bereturned to the normal control mode, storing an output frequency beforethe power failure is detected.

According to the method of controlling an alternating current motordescribed in claim 14, preferably, when the direct current intermediatevoltage is equal to the voltage before the power failure is detected orit rises during the deceleration to cause the deceleration to stop andthe alternating current motor to be returned to the normal control mode,the alternating current motor is accelerated for an arbitraryacceleration time until its output frequency is equal to the outputfrequency stored before the power failure is detected.

According to yet still another aspect of the invention described inclaim 15, there is provided a control device which controls analternating current motor. The control device includes: a powerconverter; and a power failure detecting unit which detects a powerfailure of the alternating current motor and is provided in the powerconverter. In the control device, a deceleration start instruction isoutput to an inverter unit in response to a power failure detectingsignal output from the power failure detecting unit. A first reductionrate is calculated, on the basis of a detection value and a target valueof a direct current intermediate voltage, such that a direct currentintermediate voltage is made constant during the deceleration of thealternating current motor. A second reduction rate is calculated on thebasis of a variation in the direct current intermediate voltage. A valueobtained by multiplying the two reduction rates together is controlledin a PI control manner to control a deceleration time. When the directcurrent intermediate voltage is equal to a voltage before the powerfailure is detected or it rises during the deceleration, a torqueinstruction is calculated to allow deceleration to stop and thealternating current motor to be decelerated for the deceleration time,thereby returning the alternating current motor to a normal controlmode. An electromotive torque limit value and a regenerative torquelimit value are changed on the basis of the value of the detected directcurrent intermediate voltage. A torque limit unit restricts the torqueinstruction. When the direct current intermediate voltage is equal to avoltage before the power failure is detected or it rises during thedeceleration to cause the deceleration to stop and the alternatingcurrent motor to be returned to the normal control mode, an outputfrequency before the power failure is detected is stored.

According to the method of controlling an alternating current motordescribed in claim 16, preferably, when the direct current intermediatevoltage is equal to the voltage before the power failure is detected orit rises during the deceleration to cause the deceleration to stop andthe alternating current motor to be returned to the normal control mode,the alternating current motor is accelerated for an arbitraryacceleration time until its output frequency is equal to the outputfrequency stored before the power failure is detected.

According to still yet another aspect of the invention described inclaim 17, a method of controlling an alternating current motor includesthe steps of: setting a lower limit voltage V_(U1) of a direct currentintermediate voltage required for a normal driving mode, a lowerallowable voltage V_(U0) of the direct current intermediate voltage whena lowest voltage of a power supply is input, and a power failuredetecting level voltage V_(U2) which is lower than the lower allowablevoltage V_(U0) and is higher than the lower limit voltage V_(U1); when adetection value of the direct current intermediate voltage output from adirect current intermediate voltage detecting circuit is lower than thepower failure detecting level voltage V_(U2) during the driving of thealternating current motor, decelerating the alternating current motor ata reduction rate α_(d) until the direct current intermediate voltage ishigher than the lower allowable voltage V_(U0); when the direct currentintermediate voltage is higher than the lower allowable voltage V_(U0),returning the alternating current motor to a normal control mode; andwhen the direct current intermediate voltage is higher than the lowerallowable voltage V_(U0) to cause the alternating current motor to bereturned to the normal control mode, storing an output frequency beforethe power failure is detected.

According to the method of controlling an alternating current motordescribed in claim 18, preferably, when the direct current intermediatevoltage is higher than the lower allowable voltage V_(U0) to cause thealternating current motor to be returned to the normal control mode, thealternating current motor is accelerated for an arbitrary accelerationtime until its output frequency is equal to the output frequency storedbefore the power failure is detected.

According to yet still another aspect of the invention described inclaim 19, a control device for an alternating current motor includes asequence unit which performs the functions of: setting a lower limitvoltage V_(U1) of a direct current intermediate voltage required for anormal driving mode, a lower allowable-voltage V_(U0) of the directcurrent intermediate voltage when a lowest voltage of a power supply isinput, and a power failure detecting level voltage V_(U2) which is lowerthan the lower allowable voltage V_(U0) and is higher than the lowerlimit voltage V_(U); when a detection value of the direct currentintermediate voltage output from a direct current intermediate voltagedetecting circuit is lower than the power failure detecting levelvoltage V_(U2) during the driving of the alternating current motor,decelerating the alternating current motor at a reduction rate α_(d)until the direct current intermediate voltage is higher than the lowerallowable voltage V_(U0); and when the direct current intermediatevoltage is higher than the lower allowable voltage V_(U0), returning thealternating current motor to a normal control mode. In the controldevice, when the direct current intermediate voltage is higher than thelower allowable voltage V_(U0) to cause the alternating current motor tobe returned to the normal control mode, an output frequency before thepower failure is detected is stored.

According to the control device for an alternating current motordescribed in claim 20, preferably, when the direct current intermediatevoltage is higher than the lower allowable voltage V_(U0) to cause thealternating current motor to be returned to the normal control mode, thealternating current motor is accelerated for an arbitrary accelerationtime until its output frequency is equal to the output frequency storedbefore the power failure is detected.

EFFECTS OF THE INVENTION

According to a method of controlling an alternating current motor and acontrol method therefor of the invention, it is possible to make adirect current intermediate voltage constant and thus to stably andcontinuously drive the alternating current motor even at the time of aninstantaneous power failure, without using a regenerative resistor or anapparatus for feeding back regenerative energy to a power supply, bysetting an electromotive torque limit value and a regenerative torquelimit value to restrict a torque instruction.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1]

FIG. 1 is a block diagram illustrating the structure of a control devicefor an alternating current motor according to a first embodiment of theinvention. [FIG. 2]

FIG. 2 is a diagram illustrating the relationship between a directcurrent intermediate voltage V_(PN) and electromotive and regenerativetorque limit values according to the invention.

[FIG. 3]

FIG. 3 is a diagram illustrating the relationship between a directcurrent intermediate voltage V_(PN) and electromotive and regenerativetorque limit values according to a second embodiment of the invention.

[FIG. 4]

FIG. 4 is a block diagram illustrating the structure of a control devicefor an alternating current motor according to a third embodiment of theinvention.

[FIG. 5]

FIG. 5 is a block diagram illustrating a sequence circuit for selectinga frequency instruction and an acceleration/deceleration time during thedetection of a power failure according to the third embodiment.

[FIG. 6]

FIG. 6 is a block diagram illustrating the structure of a control devicefor an alternating current motor according to a fourth embodiment of theinvention.

[FIG. 7]

FIG. 7 is a diagram illustrating the relationship between a directcurrent intermediate voltage V_(PN) and electromotive and regenerativetorque limit values according to the fourth embodiment of the invention.

[FIG. 8]

FIG. 8 is a block diagram illustrating the structure of a control devicefor an alternating current motor according to a sixth embodiment of theinvention.

[FIG. 9]

FIG. 9 is a block diagram illustrating a sequence circuit for selectingan electromotive torque limit value, a frequency instruction, and anacceleration/deceleration time during the detection of a power failureaccording to the sixth embodiment.

REFERENCE NUMERALS

1: POWER CONVERTER

11: CONVERTER UNIT

12: SMOOTHING CAPACITOR

13: INVERTER UNIT

2: ALTERNATING CURRENT MOTOR

3: TORQUE LIMIT CIRCUIT

4: VOLTAGE INSTRUCTION CALCULATING UNIT

5: SWITCHING PATTERN GENERATING CIRCUIT

6: VOLTAGE DETECTING CIRCUIT

7: TORQUE LIMIT VALUE CALCULATING CIRCUIT

8: ELECTROMAGNETIC CONTACTOR

8 a: POWER FAILURE DETECTING CONTACT

9: FREQUENCY SETTING UNIT

10: ACCELERATION/DECELERATION TIME SETTING UNIT

14: SEQUENCE CIRCUIT

15: ACCELERATION/DECELERATION TIME CONTROL CIRCUIT

16: SOFT STARTER

17: SPEED CONTROL CIRCUIT

18: FIRST SWITCHING UNIT

19: SECOND SWITCHING UNIT

20: THIRD SWITCHING UNIT

21: OUTPUT FREQUENCY STORAGE CIRCUIT BEFORE POWER FAILURE

22: EDGE TRIGGER DETECTING CIRCUIT

23, 24: OR CIRCUIT

25, 26: COMPARATOR

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, preferred embodiments of the invention will be describedbelow with reference to the accompanying drawings.

First Embodiment

FIG. 1 is a block diagram illustrating the structure of a control devicefor an alternating current (AC) motor according to a first embodiment ofthe invention. FIG. 2 is a diagram illustrating the relationship betweena DC intermediate voltage and a torque limit value of a torque limitcircuit in this embodiment.

The control device for the AC motor according to this embodimentincludes: a power converter 1 having a converter unit 11 for convertinga three-phase alternating current supplied from a three-phasealternating current power supply into a DC voltage by using a powerelement, a smoothing capacitor 12 for smoothing the converted voltage,and an inverter unit 13 for converting the DC intermediate voltage intoan AC voltage having a predetermined frequency by a PWM control manner;an AC motor 2 which is driven by an alternating current output from theinverter unit 13; a torque limit circuit 3 for controlling a torqueinstruction so as to be between a predetermined electromotive torquelimit value and a predetermined regenerative torque limit value; avoltage instruction calculating unit 4 for calculating a voltageinstruction allowing a torque to be output according to the torqueinstruction output from the torque limit circuit 3 and for outputtingthe voltage instruction; a switching pattern generating circuit 5 fordetermining a switching pattern of the power converter 1 on the basis ofthe output of the voltage instruction calculating unit 4; a voltagedetecting circuit 6 for detecting a DC intermediate voltage V_(PN),which is a voltage of the smoothing capacitor 12; a torque limit valuecalculating circuit 7 for calculating an electromotive torque limitvalue T_(L) and a regenerative torque limit value T_(GL), on the basisof the DC intermediate voltage V_(PN) output from the voltage detectingcircuit 6 and for setting the calculated values to the torque limitcircuit 3.

In general, torque output by the AC motor 2 is controlled by a machineconnected to the AC motor 2, the AC motor 2, or the power converter 1,in response to a predetermined torque instruction. Therefore, anelectromotive torque limit value T_(L0) and a regenerative torque limitvalue T_(GL0) are set in advance. Thus, the AC motor 2 is controlledwithin the torque limit.

However, when the AC motor 2 is rapidly decelerated or when an outputfrequency is raised by a gravity load or a mechanical load, the AC motor2 generates a regenerative torque, and regenerative energy is fed backto the AC motor 2, causing the DC intermediate voltage V_(PN) to rise.When the DC intermediate voltage V_(PN) is higher than a predeterminedlevel, the AC motor may be non-controllable, or a circuit may be broken.Therefore, it is necessary to control the DC intermediate voltage V_(PN)so as not to rise beyond the predetermined level.

Therefore, in general, a resistor is connected in parallel to thesmoothing capacitor 12 to provide a regenerative resistor for consumingpower, or an apparatus for feeding back regenerative energy to a powersupply is used. However, in the invention, the rise of the DCintermediate voltage V_(PN) is controlled without using the regenerativeresistor or the apparatus for feeding back the regenerative energy tothe power supply. In addition, the electromotive torque limit value andthe regenerative torque limit value are changed according to the levelof the DC intermediate voltage V_(PN) such that the AC motor iscontinuously driven when the DC intermediate voltage V_(PN) is lowereddue to, for example, an instantaneous power failure.

More specifically, as an example of a method of setting the torque limitvalue, the relationship between the DC intermediate voltage V_(PN) andthe torque limit value is defined as shown in FIG. 2, and theelectromotive torque limit value and the regenerative torque limit valueare controlled according to the level of the DC intermediate voltageV_(PN).

When the DC intermediate voltage V_(PN) is higher than a voltageV_(UVH), the electromotive torque limit value is set to a predeterminedelectromotive torque limit value T_(L0). On the other hand, when the DCintermediate voltage V_(PN) is lower than a voltage V_(UVL), theelectromotive torque limit value is set to zero so as not to generateelectromotive torque. When the DC intermediate voltage V_(PN) is higherthan the voltage V_(UVL) and lower than the voltage V_(UVH), theelectromotive torque limit value rises from zero to a predeterminedtorque limit value T_(L1) in proportion to the DC intermediate voltageV_(PN). In general, the range of the electromotive torque limit value isgradually narrowed, as the DC intermediate voltage V_(PN) becomes lowerthan the voltage V_(UVH). Therefore, this setting prevents the DCintermediate voltage from being further lowered due to the lowering ofthe DC intermediate voltage V_(PN). Thus, the DC intermediate voltageV_(PN) is not sharply lowered, which makes it possible to continuouslydrive the AC motor.

Meanwhile, when the DC intermediate voltage V_(PN) is lower than avoltage V_(OVL), the regenerative torque limit value turns to apredetermined regenerative torque limit value T_(GL0). On the otherhand, when the DC intermediate voltage V_(PN) is higher than a voltageV_(OVH), the regenerative torque limit value is set to zero so as not togenerate regenerative torque. Between the voltage V_(OVL) and thevoltage V_(OVH), the regenerative torque limit value is lowered from apredetermined torque limit value T_(GL1) to zero in proportion to the DCintermediate voltage V_(PN). In general, the range of the regenerativetorque limit value is gradually narrowed, as the DC intermediate voltageV_(PN) becomes higher than the voltage V_(OVL). Therefore, this settingprevents the DC intermediate voltage from being further raised due to arise in the DC intermediate voltage V_(PN). Thus, the DC intermediatevoltage V_(PN) does not sharply rise, which makes it possible tocontinuously drive the AC motor.

In this way, it is possible to control the DC intermediate voltage to beconstant at the time of an instantaneous power failure and thus tocontinuously drive the AC motor smoothly.

Further, when the DC intermediate voltage V_(PN) is lower than thevoltage V_(OVL) during rapid deceleration, the AC motor is deceleratedat a reduction rate α_(d) in the response to a rapid decelerationinstruction. On the other hand, when the DC intermediate voltage V_(PN)is higher than the voltage V_(OVL), the range of the regenerative torquelimit value becomes narrower as the DC intermediate voltage becomeshigher. Therefore, the reduction rate α_(d) is gradually lowered tocause the AC motor to be decelerated at a regenerative level which canbe absorbed by the power converter. As a result, the AC motor can besmoothly decelerated.

Furthermore, in a structure in which a load side raises the speed of theAC motor, until the DC intermediate voltage V_(PN) rises up to thevoltage V_(OVL), a regenerative torque is output so as to ensure speedaccuracy, that is, speed control is performed. When the DC intermediatevoltage V_(PN) is higher than the voltage V_(OVL), the range of theregenerative torque limit value is narrowed such that the DCintermediate voltage does not further rise. Then, the speed of the ACmotor is lowered corresponding to the reduced value, reducing in thedeterioration of speed accuracy. However, according to this embodiment,it is possible to prevent the DC intermediate voltage from being furtherraised and thus to smoothly and continuously drive the AC motor. Whenthe DC intermediate voltage is lowered, the range of the regenerativetorque limit value is immediately widened, and then the regenerativetorque occurs, which makes it possible to make the speed of the AC motorconstant.

In the above-mentioned structure in which the regenerative torque limitvalue is controlled according to the level of the DC intermediatevoltage, when the rise of the DC intermediate voltage occurs in thepower converter not including the regenerative resistor or the apparatusfor feeding back the regenerative energy to the power supply, the speedaccuracy is deteriorated, but an excessive rise in voltage does notoccur, which makes it possible to smoothly and continuously drive the ACmotor.

In this embodiment, FIG. 2 shows the relationship between the DCintermediate voltage and the electromotive and regenerative torque limitvalues. The electromotive torque limit values T_(L1) and T_(L0) may beequal to each other, or the regenerative torque limit values T_(GL1) andT_(GL0) may be equal to each other. In addition to the relationship inwhich the electromotive torque limit value and the regenerative torquelimit value are proportional to the DC intermediate voltage, anyrelationship may be used as long as the following is satisfied: when theDC intermediate voltage becomes lower, the electromotive torque limitvalue approximates zero, and when the DC intermediate voltage becomeshigher, the regenerative torque limit value approximates zero. Further,the following method is also preferable: from the relationship betweenthe inertia of a machine or the capacitance of a smoothing capacitor andtorque, the next level of the DC intermediate voltage is estimated tonarrow the ranges of the electromotive and regenerative torque limitvalues.

Second Embodiment

A second embodiment of the invention using the control device for the ACmotor shown in FIG. 2 will be described below. In this embodiment, thefollowing method of controlling an AC motor is provided. A lower limitvoltage V_(U1) of the DC intermediate voltage V_(PN) required to drivethe AC motor 2, a lower limit allowable voltage V_(U0) of the DCintermediate voltage V_(PN) equal to a lower limit voltage of the powersupply, and a power failure detecting level voltage V_(U2) lower thanthe lower limit allowable voltage V_(U0) and higher than the lower limitvoltage V_(U1) are set in advance. When the DC intermediate voltageV_(PN) output from the DC intermediate voltage detecting circuit 6 islower than the power failure detecting level voltage V_(U2) during thedriving of the AC motor, the AC motor is decelerated at a predeterminedreduction rate α_(d) until the DC intermediate voltage V_(PN) exceedsthe lower limit allowable voltage V_(U0). When the DC intermediatevoltage V_(PN) exceeds the lower limit allowable voltage V_(U0), thealternating current motor is returned to a normal control mode.

More specifically, the relationship between the DC intermediate voltageV_(PN) and a torque limit value shown in FIG. 3 is defined in the torquelimit circuit 3. An electromotive torque limit value is set to zero whenthe DC intermediate voltage V_(PN) is lower than the lower limit voltageV_(U1). When the DC intermediate voltage V_(PN) is between the lowerlimit voltage V_(U1) and the lower limit allowable voltage V_(U0), theelectromotive torque limit value gradually increase from zero to apredetermined value. When the DC intermediate voltage V_(PN) is higherthan the lower limit allowable voltage V_(U0), the electromotive torquelimit value is set to a predetermined value. When the DC intermediatevoltage V_(PN) is lower than a low voltage level V_(U3) between thepower failure detecting level voltage V_(U2) and the lower limit voltageV_(U1), the regenerative torque limit value is set to a predeterminedvalue. When the DC intermediate voltage V_(PN) is between the lowvoltage level V_(U3) and the lower limit allowable voltage V_(U0), theregenerative torque limit value gradually decreases from thepredetermined value to zero. When the DC intermediate voltage V_(PN) islower than the lower limit allowable voltage V_(U0), the regenerativetorque limit value is zero.

In this embodiment, control is performed at the time of power failuresuch that the electromotive and regenerative torque limit values arelimited according to the level of the DC intermediate voltage V_(PN),which makes it possible to control the DC intermediate voltage to beconstant and thus to smoothly and continuously drive the AC motor.

In FIG. 3, the low voltage level V_(U3) is set to an arbitrary value.However, the low voltage level V_(U3) may be equal to the power failuredetecting level voltage V_(U2). In addition, the torque limit valuevaries in proportion to the DC intermediate voltage V_(PN). However, anarbitrary relationship may be established therebetween.

In this embodiment, when the DC intermediate voltage V_(PN) exceeds thelower limit allowable voltage V_(U0), the alternating current motor isreturned to a normal control mode. However, at the time of powerfailure, reduction control is performed such that the DC intermediatevoltage V_(PN) is made constant, causing the AC motor to be driven at anoutput frequency lower than the output frequency before the powerfailure. For this reason, the AC motor is rapidly accelerated inresponse to an output frequency instruction, a large torque instructionis output to make the power failure conditions again, or shock occursdue to a sharp variation in the speed of the AC motor, making itdifficult to perform smooth and continuous driving. Therefore, accordingto this embodiment of the invention, the output frequency when the powerfailure is detected is stored in advance. When the DC intermediatevoltage V_(PN) exceeds the lower limit allowable voltage V_(U0) to causethe alternating current motor to be returned to the normal control mode,the AC motor is accelerated for only a predetermined acceleration timeuntil its output frequency is equal to the stored output frequency.Alternatively, the output frequency when the power failure is detectedis stored in advance. When the DC intermediate voltage V_(PN) exceedsthe lower limit allowable voltage V_(U0) to cause the alternatingcurrent motor to be returned to the normal control mode, a torqueinstruction is restricted by a predetermined electromotive torque limitvalue until the output frequency of the AC motor is equal to the storedoutput frequency, which prevents the AC motor from being rapidlyaccelerated.

In this way, even when a power failure is detected, it is possible tosmoothly and continuously drive the AC motor.

Third Embodiment

FIG. 4 is a block diagram illustrating the structure of a control devicefor an AC motor according to a third embodiment of the invention. FIG. 5is a block diagram illustrating a sequence circuit for selectingacceleration/deceleration time and a frequency instruction during thedetection of a power failure.

In FIG. 4, the control device for the AC motor according to thisembodiment includes: a power converter 1 having a converter unit 11 forconverting a three-phase alternating current supplied from a three-phasealternating current power supply (not shown) through an electromagneticcontactor 8 into a DC voltage by using a power element, a smoothingcapacitor 12 for smoothing the converted voltage, and an inverter unit13 for converting a DC intermediate voltage into an AC voltage having apredetermined frequency by a PWM control manner; an AC motor 2 which isdriven by an alternating current output from the inverter unit 13; avoltage instruction calculating unit 4 for calculating a voltageinstruction on the basis of a frequency output from a soft starter; aswitching pattern generating circuit 5 for determining a switchingpattern of the power converter 1 on the basis of the output of thevoltage instruction calculating unit 4; a voltage detecting circuit 6for detecting a DC intermediate voltage V_(PN), which is a voltage ofthe smoothing capacitor 12; a frequency setting unit 9 for setting anoutput frequency; an acceleration/deceleration time setting unit 10 forsetting acceleration/deceleration time when the AC motor 2 isaccelerated or decelerated from a stop state until the maximum outputfrequency is output; a sequence circuit 14 for detecting a powerfailure, on the basis of a power failure detecting signal from a powerfailure detecting contact 8 a of the electromagnetic contactor 8, andfor setting the frequency instruction and the deceleration time duringthe detection of the power failure; an acceleration/deceleration timecontrol circuit 15 for controlling the deceleration time such that theDC intermediate voltage V_(PN) is made constant by PI control during thedetection of the power failure and for controlling the acceleration timeafter the supply of power resumes; and a soft starter 16 for raising orlowering the output frequency for a predeterminedacceleration/deceleration time.

As shown in FIG. 5, in the sequence circuit 14, a first switching unit18 performs switching between a general frequency instruction outputfrom the frequency setting unit 9 and a frequency instruction output atthe time of a power failure to output the frequency instruction to thesoft starter 16, in response to a power failure detecting signal outputbased on a low voltage detection output signal output from the voltagedetecting circuit 6 (which is output when the DC intermediate voltageV_(PN) is lower than a set UV level of 1) or a power failure outputsignal output from the power failure detecting contact 8 a of theelectromagnetic contactor 8. A second switching unit 19 selects one of ageneral deceleration time output from the acceleration/deceleration timesetting unit 10 and a deceleration time during the detection of a powerfailure and outputs the selected time to the acceleration/decelerationtime control circuit 15. An edge trigger detecting circuit 22 detects apoint of time when the power failure detecting signal turns from ‘0’ to‘1’. An output frequency storage circuit 21 before a power failurestores an output frequency when the edge trigger detecting circuit 22detects the power failure. A third switching unit 20 selects one of ageneral acceleration time output from the acceleration/deceleration timesetting unit 10 and an acceleration time after the supply of powerresumes and outputs the selected time to the acceleration/decelerationtime control circuit 15 when the power failure detecting signal ismaintained at the level of ‘1’ and until the output frequency outputfrom the soft contactor is equal to the output frequency stored in theoutput frequency storage circuit 21 before the power failure when thepower failure detecting signal is at the level of ‘0’. In FIG. 5,reference numerals 23 and 24 denote OR circuits, and reference numerals25 and 26 indicate comparators.

Next, a description will be made below of a method of controllingcontinuous driving during the detection of a power failure in thecontrol device for the AC motor according to this embodiment.

When an instantaneous power failure occurs in an AC power supply, theelectromagnetic contactor 8 (FIG. 4) is opened, which causes the powerfailure detecting contact turns to ‘1’ in level or the DC intermediatevoltage of the smoothing capacitor 12 to be lower than a UV level1V_(UV1). Then, the voltage detecting circuit 6 for detecting the DCintermediate voltage detects the power failure. When the power failureis detected, the power failure detecting contact signal turns to ‘1’ inlevel, or the low voltage detecting signal is turned to ‘1’ in level bythe DC intermediate voltage V_(PN) detected by the voltage detectingcircuit 6, as shown in FIG. 5. Then, the signals are input to thesequence circuit 14. In the sequence circuit 14, when the power failuredetecting contact signal is at the level of ‘1’ or the low voltagedetecting signal is at the level of ‘1’, the power failure detectingsignal turns to ‘1’ in level. The frequency instruction is switched to 0by the first and second switching units 18 and 19, so that thedeceleration time is switched to the set deceleration time during thedetection of a power failure, and the output frequency when the powerfailure is detected is stored in the output frequency storage circuit 12before the power failure. When the power converter 1 for controlling thedriving of the AC motor 2 is changed from a normal driving mode to adeceleration mode, rotational energy corresponding to the reduction rateof the AC motor is transformed into regenerative power by the inverterunit 13, and the smoothing capacitor 12 forming a DC intermediatecircuit is charged, causing the terminal voltage thereof to rise.

When the power failure detecting signal turns to ‘1’ in level, theacceleration/deceleration time control circuit 15 controls a reductionrate by a PI control manner, referring to the level of the DCintermediate voltage V_(PN) and the variation of the DC intermediatevoltage, such that the DC intermediate voltage V_(PN) detected by thevoltage detecting circuit 6 is equal to an input voltage set value×1.35.In this way, the rotational speed of the AC motor 2 is lowered notrapidly but slowly, which enables the motor to be continuously drivenduring the instantaneous power failure. When a rise in the DCintermediate voltage V_(PN) is detected, or when the DC intermediatevoltage V_(PN) is higher than the voltage before the power failure isdetected, deceleration stops. When the supply of power is resumed by theAC power supply of the inverter unit 13, the power failure detectingcontact 8 a of the electromagnetic contactor 8 is closed, and the DCintermediate voltage V_(PN) is higher than the low voltage detectinglevel, so that the AC motor is returned to the normal driving mode. TheAC motor is driven for the acceleration time from theacceleration/deceleration time setting unit 10 after the supply of powerresumes, until the output frequency output from the soft starter isequal to the output frequency stored in the output frequency storagecircuit 21 before the power failure. When the output frequency outputfrom the soft starter is equal to the output frequency stored in theoutput frequency storage circuit 21 before the power failure, the ACmotor is accelerated or decelerated for the acceleration/decelerationtime generally set, until the output frequency thereof is equal to afrequency set value. In this way, it is possible to continuously drivean AC motor when a power failure is detected.

Fourth Embodiment

FIG. 6 is a block diagram illustrating the structure of a control devicefor an AC motor according to a fourth embodiment of the invention.

In FIG. 6, the control device for the AC motor according to thisembodiment includes: a power converter 1 having a converter unit 11 forconverting a three-phase alternating current supplied from a three-phasealternating current power supply (not shown) through an electromagneticcontactor 8 into a DC voltage by using a power element, a smoothingcapacitor 12 for smoothing the converted voltage, and an inverter unit13 for converting a DC intermediate voltage into an AC voltage having apredetermined frequency by a PWM control manner; an AC motor 2 which isdriven by an alternating current output from the inverter unit 13; atorque limit circuit 3 for controlling a torque instruction so as to bebetween a predetermined electromotive torque limit value and apredetermined regenerative torque limit value; a voltage instructioncalculating unit 4 for calculating a voltage instruction allowing torqueto be output according to the torque instruction output from the torquelimit circuit 3 and for outputting the voltage instruction; a switchingpattern generating circuit 5 for determining a switching pattern of thepower converter 1 on the basis of the output of the voltage instructioncalculating unit 4; a voltage detecting circuit 6 for detecting a DCintermediate voltage V_(PN), which is a voltage of the smoothingcapacitor 12; a torque limit value calculating circuit 7 for calculatingan electromotive torque limit value T_(L) and a regenerative torquelimit value T_(GL), on the basis of the DC intermediate voltage V_(PN)output from the voltage detecting circuit 6 and for setting thecalculated values to the torque limit circuit 3; a frequency settingunit 9 for setting an output frequency; an acceleration/decelerationtime setting unit 10 for setting acceleration/deceleration time when theAC motor 2 is accelerated or decelerated from a stop state until themaximum output frequency is output; a sequence circuit 14 for detectinga power failure, on the basis of a power failure detecting signal from apower failure detecting contact 8 a of the electromagnetic contactor 8,and for setting the frequency instruction and the deceleration timeduring the detection of the power failure; an acceleration/decelerationtime control circuit 15 for controlling the deceleration time such thatthe DC intermediate voltage V_(PN) is made constant by PI control duringthe detection of the power failure and for controlling the accelerationtime after the supply of power resumes; a soft starter 16 for raising orlowering the output frequency for a predeterminedacceleration/deceleration time; and a speed control circuit 17 foroutputting a torque instruction on the basis of the frequency outputfrom the soft starter 16.

FIG. 5 is a block diagram illustrating a sequence circuit according tothe third embodiment for selecting acceleration/deceleration time and afrequency instruction during the detection of a power failure. In FIG.5, reference numeral 20 denotes a third switching unit, and referencenumeral 21 denotes an output frequency storage circuit before a powerfailure. Reference numeral 22 denotes an edge trigger detecting circuit,and reference numerals 23 and 24 denote OR circuits. Reference numerals25 and 26 denote comparators.

As shown in FIG. 5, in the sequence circuit 14, a first switching unit18 performs switching between a general frequency instruction outputfrom the frequency setting unit 9 and a frequency instruction output atthe time of a power failure to output the frequency instruction to thesoft starter 16, on the basis of the power failure detecting signaloutput based on a low voltage detecting output signal output from thevoltage detecting circuit 6 (which is output when the DC intermediatevoltage V_(PN) is lower than a set UV level of 1) or a power failureoutput signal output from the power failure detecting contact 8 a of theelectromagnetic contactor 8. A second switching unit 19 selects one of ageneral deceleration time output from the acceleration/deceleration timesetting unit 10 and a deceleration time during the detection of a powerfailure and outputs the selected time to the acceleration/decelerationtime control circuit 15. The edge trigger detecting circuit 22 detects apoint of time when the power failure detecting signal turns from ‘0’ to‘1’. The output frequency storage circuit 21 before the power failurestores an output frequency when the edge trigger detecting circuit 22detects the power failure. The third switching unit 20 selects one of ageneral acceleration time output from the acceleration/deceleration timesetting unit 10 and an acceleration time after the supply of powerresumes and outputs the selected time to the acceleration/decelerationtime control circuit 15 when the power failure detecting signal ismaintained at the level of ‘1’ and until the output frequency outputfrom the soft contactor is equal to the output frequency stored in theoutput frequency storage circuit 21 before the power failure when thepower failure detecting signal is at the level of ‘0’.

Next, a description will be made below of a method of controllingcontinuous driving during the detection of a power failure in thecontrol device for the AC motor according to this embodiment.

When an instantaneous power failure occurs in an AC power supply, theelectromagnetic contactor 8 (FIG. 6) is opened, which causes the powerfailure detecting contact turns to ‘1’ in level or the DC intermediatevoltage of the smoothing capacitor 12 to be lower than a UV level1V_(UV1). Then, the voltage detecting circuit 6 for detecting the DCintermediate voltage detects the power failure. When the power failureis detected, the power failure detecting contact signal turns to ‘1’ inlevel, or the low voltage detecting signal is turned to ‘1’ in level bythe DC intermediate voltage V_(PN) detected by the voltage detectingcircuit 6, as shown in FIG. 5. Then, the signals are input to thesequence circuit 14. In the sequence circuit 14, when the power failuredetecting contact signal is at the level of ‘1’ or the low voltagedetecting signal is at the level of ‘1’, the power failure detectingsignal turns to ‘1’ in level. The frequency instruction is switched to 0by the first and second switching units 18 and 19, so that thedeceleration time is switched to the set deceleration time during thedetection of a power failure, and the output frequency when the powerfailure is detected is stored in the output frequency storage circuit 12before the power failure. When the power converter 1 for controlling thedriving of the AC motor 2 is changed from a normal driving mode to adeceleration mode, rotational energy corresponding to the reduction rateof the AC motor is transformed into regenerative power by the inverterunit 13, and the smoothing capacitor 12 forming a DC intermediatecircuit is charged, causing the terminal voltage thereof to rise.

When the power failure detecting signal turns to ‘1’ in level, theacceleration/deceleration time control circuit 15 controls a reductionrate by a PI control manner, referring to the level of the DCintermediate voltage V_(PN) and the variation of the DC intermediatevoltage, such that the DC intermediate voltage V_(PN) detected by thevoltage detecting circuit 6 is equal to an input voltage set value×1.35.In this way, the rotational speed of the AC motor 2 is lowered notrapidly but slowly, which enables the motor to be continuously drivenduring the instantaneous power failure. When a rise in the DCintermediate voltage V_(PN) is detected, or when the DC intermediatevoltage V_(PN) is higher than the voltage before the power failure isdetected, deceleration stops. When the supply of power is resumed by theAC power supply of the inverter unit 13, the power failure detectingcontact 8 a of the electromagnetic contactor 8 is closed, and the DCintermediate voltage V_(PN) is higher than the low voltage detectinglevel, so that the AC motor is returned to the normal driving mode. TheAC motor is driven for the acceleration time from theacceleration/deceleration time setting unit 10 after the supply of powerresumes, until the output frequency output from the soft starter isequal to the output frequency stored in the output frequency storagecircuit 21 before the power failure. When the output frequency outputfrom the soft starter is equal to the output frequency stored in theoutput frequency storage circuit 21 before the power failure, the ACmotor is accelerated or decelerated for the acceleration/decelerationtime generally set, until the output frequency thereof is equal to afrequency set value. In this way, it is possible to continuously drivean AC motor when a power failure is detected.

Fifth Embodiment

In a fifth embodiment, in addition to the function described in thefourth embodiment, the speed control circuit 17 for making a frequencyF_(out) output from the soft starter 16 shown in FIG. 6 equal to thespeed of the AC motor 2 creates a torque instruction T_(ref). When theAC motor 2 is controlled by the torque instruction, the DC intermediatevoltage is oscillated due to the inertia of a load, a reduction ratecoefficient of the acceleration/deceleration time control circuit, or aset value of a PI controller, or the AC motor 2 repeatedly performs adecelerating operation caused by a power failure and an acceleratingoperation by normal control, which makes it difficult to perform smoothand continuous driving.

FIG. 7 shows the relationship between the DC intermediate voltage V_(PN)and a torque limit value when the power failure signal is maintained atthe level of ‘1’. The electromotive torque limit value and theregenerative torque limit value are controlled on the basis of the levelof the DC intermediate voltage V_(PN).

That is, when the DC intermediate voltage V_(PN) is higher than a levelV_(UV1), the electromotive torque limit value is set to a predeterminedelectromotive torque limit value T_(L0). On the other hand, when the DCintermediate voltage V_(PN) is lower than a second UV level V_(UV2)where the power converter 1 cannot be operated, the electromotive torquelimit value is set to zero so as not to generate electromotive torque.When the DC intermediate voltage V_(PN) is higher than the level V_(UV2)and lower than the voltage V_(UV1), the electromotive torque limit valuerises from zero to a predetermined torque limit value T_(L0) inproportion to the DC intermediate voltage V_(PN). In general, the rangeof the electromotive torque limit value is gradually narrowed, as the DCintermediate voltage V_(PN) becomes lower than the level V_(UV1).Therefore, this setting prevents the DC intermediate voltage from beingfurther lowered due to the lowering of the DC intermediate voltageV_(PN).

Meanwhile, when the DC intermediate voltage V_(dc) is lower than a thirdUV level V_(UV3), the regenerative torque limit value turns to apredetermined regenerative torque limit value T_(GL0), On the otherhand, when the DC intermediate voltage V_(PN) is higher than a fourth UVlevel V_(UV4), the regenerative torque limit value is set to zero so asnot to generate regenerative torque. When the DC intermediate voltageV_(PN) is higher than the level V_(UV3) and the lower than the levelV_(UV4), the regenerative torque limit value is lowered from thepredetermined torque limit value T_(GL0) to zero in proportion to the DCintermediate voltage V_(PN). In general, the range of the regenerativetorque limit value is gradually narrowed, as the DC intermediate voltageV_(PN) becomes higher than the level V_(UV3). Therefore, this settingprevents the DC intermediate voltage from being further raised due to arise in the DC intermediate voltage V_(PN).

In this way, it is possible to control the torque instruction by settingthe electromotive torque limit value and the regenerative torque limitvalue on the basis of the level of the DC intermediate voltage V_(PN).Therefore, it is possible to stably control the DC intermediate voltageV_(PN), for example, at the time of an instantaneous power failure andto smoothly and continuously drive the AC motor.

In FIG. 7, the third UV level V_(UV3) is set to an arbitrary value.However, the third UV level V_(UV3) may be equal to the second UV levelV_(UV2). The fourth UV level V_(UV4) is set to an arbitrary value.However, the fourth UV level V_(UV4) may be equal to the first UV levelV_(UV1). The torque limit value is changed in proportion to the DCintermediate voltage V_(PN). However, any relationship may beestablished therebetween.

Sixth Embodiment

FIG. 8 is a block diagram illustrating the structure of a control devicefor an alternating current motor according to a sixth embodiment of theinvention. FIG. 9 is a block diagram illustrating a sequence circuit forselecting an acceleration/deceleration time, a frequency instruction,and an electromotive torque limit value during the detection of a powerfailure.

In FIG. 8, the control device for the AC motor according to thisembodiment includes: a power converter 1 having a converter unit 11 forconverting a three-phase alternating current supplied from a three-phasealternating current power supply (not shown) through an electromagneticcontactor 8 into a DC voltage by using a power element, a smoothingcapacitor 12 for smoothing the converted voltage, and an inverter unit13 for converting a DC intermediate voltage into an AC voltage having apredetermined frequency by a PWM control manner; an AC motor 2 which isdriven by an alternating current output from the inverter unit 13; atorque limit circuit 3 for controlling a torque instruction so as to bebetween a predetermined electromotive torque limit value and apredetermined regenerative torque limit value; a voltage instructioncalculating unit 4 for calculating a voltage instruction allowing torqueto be output according to the torque instruction output from the torquelimit circuit 3 and for outputting the voltage instruction; a switchingpattern generating circuit 5 for determining a switching pattern of thepower converter 1 on the basis of the output of the voltage instructioncalculating unit 4; a voltage detecting circuit 6 for detecting a DCintermediate voltage V_(PN), which is a voltage of the smoothingcapacitor 12; a torque limit value calculating circuit 7 for calculatingthe DC intermediate voltage V_(PN) output from the voltage detectingcircuit 6 and for setting an electromotive torque limit value T_(L) anda regenerative torque limit value T_(GL) output from a sequence circuit14; a frequency setting unit 9 for setting an output frequency; anacceleration/deceleration time setting unit 10 for setting anacceleration/deceleration time when the AC motor 2 is accelerated ordecelerated from a stop state until the maximum output frequency isoutput; the sequence circuit 14 for detecting a power failure, on thebasis of a power failure detecting signal from a power failure detectingcontact 8 a of the electromagnetic contactor 8, and for setting thefrequency instruction and the deceleration time during the detection ofthe power failure; an acceleration/deceleration time control circuit 15for controlling the deceleration time such that the DC intermediatevoltage V_(PN) is made constant by PI control during the detection ofthe power failure; a soft starter 16 for raising or lowering the outputfrequency for a predetermined acceleration/deceleration time; and aspeed control circuit 17 for outputting a torque instruction on thebasis of the frequency output from the soft starter 16.

As shown in FIG. 9, in the sequence circuit 14, a first switching unit18 performs switching between a general frequency instruction outputfrom the frequency setting unit 9 and a frequency instruction output atthe time of a power failure to output the frequency instruction to thesoft starter 16, in response to a power failure detecting signal outputbased on a low voltage detection output signal output from the voltagedetecting circuit 6 (which is output when the DC intermediate voltageV_(PN) is lower than a set UV level of 1) or a power failure outputsignal output from the power failure detecting contact 8 a of theelectromagnetic contactor 8. A second switching unit 19 selects one of ageneral deceleration time output from the acceleration/deceleration timesetting unit 10 and a deceleration time during the detection of thepower failure and outputs the selected time to theacceleration/deceleration time control circuit 15. An edge triggerdetecting circuit 22 detects a point of time when the power failuredetecting signal turns from ‘0’ to ‘1’. An output frequency storagecircuit 21 before the power failure stores an output frequency when theedge trigger detecting circuit 22 detects the power failure. A thirdswitching unit 20 selects one of a general electromotive torque limitvalue and an electromotive torque limit value after the supply of powerresumes and outputs the selected value to the torque limit valuecalculating circuit 7 when the power failure detecting signal ismaintained at the level of ‘1’ and until the output frequency outputfrom the soft contactor is equal to the output frequency stored in theoutput frequency storage circuit 21 before the power failure when thepower failure detecting signal is at the level of ‘0’. In FIG. 9,reference numerals 23 and 24 denote OR circuits, and reference numerals25 and 26 indicate comparators.

Next, a description will be made below of a method of controllingcontinuous driving during the detection of a power failure in thecontrol device for the AC motor according to this embodiment.

When an instantaneous power failure occurs in an AC power supply, theelectromagnetic contactor 8 (FIG. 8) is opened, which causes the powerfailure detecting contact turns to ‘1’ in level or the DC intermediatevoltage of the smoothing capacitor 12 to be lower than a UV level1V_(UV1). Then, the voltage detecting circuit 6 for detecting the DCintermediate voltage detects the power failure. When the power failureis detected, the power failure detecting contact signal turns to ‘1’ inlevel, or the low voltage detecting signal is turned to ‘1’ in level bythe DC intermediate voltage V_(PN) detected by the voltage detectingcircuit 6, as shown in FIG. 9. Then, the signals are input to thesequence circuit 14. In the sequence circuit 14, when the power failuredetecting contact signal is at the level of ‘1’ or the low voltagedetecting signal is at the level of ‘1’, the power failure detectingsignal turns to ‘1’ in level. The frequency instruction is switched to 0by the first and second switching units 18 and 19, so that thedeceleration time is switched to the set deceleration time during thedetection of a power failure, and the output frequency when the powerfailure is detected is stored in the output frequency storage circuit 12before the power failure. When the power converter 1 for controlling thedriving of the AC motor 2 is changed from a normal driving mode to adeceleration mode, rotational energy corresponding to the reduction rateof the AC motor is transformed into regenerative power by the inverterunit 13, and the smoothing capacitor 12 forming a DC intermediatecircuit is charged, causing the terminal voltage thereof to rise. Whenthe power failure detecting signal turns to Ill in level, theacceleration/deceleration time control circuit 15 controls a reductionrate by a PI control manner, referring to the level of the DCintermediate voltage V_(PN) and the variation of the DC intermediatevoltage, such that the DC intermediate voltage V_(PN) detected by thevoltage detecting circuit 6 is equal to an input voltage set value×1.35.In this way, the rotational speed of the AC motor 2 is lowered notrapidly but slowly, which enables the motor to be continuously drivenduring the instantaneous power failure. When a rise in the DCintermediate voltage V_(PN) is detected, or when the DC intermediatevoltage V_(PN) is higher than the voltage before the power failure isdetected, deceleration stops. When the supply of power is resumed by theAC power supply of the inverter unit 13, the power failure detectingcontact 8 a of the electromagnetic contactor 8 is closed, and the DCintermediate voltage V_(PN) is higher than the low voltage detectinglevel, so that the AC motor is returned to the normal driving mode. Theelectromotive torque limit value is selected as the electromotive torquelimit value after the supply of power resumes, until the outputfrequency output from the soft starter is equal to the output frequencystored in the output frequency storage circuit 21 before the powerfailure. The torque limit value calculating circuit 7 defines therelationship between the DC intermediate voltage V_(PN) and the torquelimit value shown in FIG. 7, and controls the electromotive torque limitvalue and the regenerative torque limit value on the basis of the levelof the DC intermediate voltage V_(PN). Therefore, the AC motor is drivenby one of the electromotive torque limit value output from the sequencecircuit 14 and the electromotive limit value determined by the level ofthe DC intermediate voltage V_(PN) having a smaller value. When theoutput frequency output from the soft starter is equal to the outputfrequency stored in the output frequency storage circuit 21 before thepower failure, the AC motor is accelerated or decelerated by one of theelectromotive torque limit value generally set and the electromotivelimit value determined by the level of the DC intermediate voltageV_(PN) having a smaller value, until the output frequency thereofapproaches is equal to a frequency set value. In this way, it ispossible to continuously drive an AC motor when a power failure isdetected.

INDUSTRIAL APPLICABILITY

According to the invention, it is possible to continuously drive an ACmotor at the time of an instantaneous power failure. An electromotivetorque limit value and a regenerative torque limit value are set on thebasis of the level of a DC intermediate voltage (the smoothing capacitor12) to control a torque instruction, which makes it possible to controlthe DC intermediate voltage to be constant, without using a regenerativeresistor or an apparatus for feeding back regenerative energy to a powersupply circuit, and thus to stably and continuously drive the AC motorat the time of the instantaneous power failure. For example, in atextile machine, speed control is performed such that a fixedrelationship is established among the speeds of a plurality of motors,and thus it is possible to prevent thread from being cut due to the freerun (coasting) of an AC motor at the time of an instantaneous powerfailure.

1. A method of controlling an alternating current motor and a powerconverter including: a converter unit for converting an alternatingcurrent voltage supplied from an alternating current power supply into adirect current voltage, and a smoothing capacitor for smoothing theconverted direct current voltage, and an inverter unit for converting adirect current intermediate voltage into an alternating current voltagehaving a frequency corresponding to a torque instruction by a PWMcontrol manner, by setting an electromotive torque limit value and aregenerative torque limit value in advance, and by restricting thetorque instruction with the torque limit values to generate a PWMswitching pattern to be output to the inverter unit, the methodcomprising the steps of: allowing a power failure detecting unitprovided in the power converter to detect a power failure of thealternating current power source; outputting a deceleration startinstruction to the inverter unit in response to a power failuredetecting signal output from the power failure detecting unit;calculating a first reduction rate, on the basis of a detection valueand a target value of the direct current intermediate voltage, such thatthe direct current intermediate voltage is made constant during thedeceleration of the alternating current motor; calculating a secondreduction rate on the basis of a variation in the direct currentintermediate voltage; controlling a deceleration time by controlling avalue obtained by multiplying the two reduction rates together in a PIcontrol manner; calculating the torque instruction to allow thealternating current motor to be decelerated for the deceleration time;changing the electromotive torque limit value and the regenerativetorque limit value on the basis of the value of the detected directcurrent intermediate voltage; stopping the deceleration when the directcurrent intermediate voltage is equal to a voltage before the powerfailure is detected or it rises during the deceleration; and when thealternating current motor is returned to a normal control mode, storingan output frequency before the power failure is detected.
 2. The methodof controlling an alternating current motor according to claim 1,wherein when the direct current intermediate voltage is equal to thevoltage before the power failure is detected or it rises during thedeceleration, the deceleration stops, and when the alternating currentmotor is returned to the normal control mode, the alternating motor isaccelerated for an arbitrary acceleration time until its outputfrequency is equal to the output frequency stored before the powerfailure is detected.
 3. The method of controlling an alternating currentmotor according to claim 1, wherein when the direct current intermediatevoltage is equal to the voltage before the power failure is detected orit rises during the deceleration, the deceleration stops, and when thealternating current motor is returned to the normal control mode, atorque limit unit restricts the torque instruction with an arbitraryelectromotive torque limit value until the output frequency of thealternating current motor is equal to the output frequency stored beforethe power failure is detected.
 4. A control device which controls analternating current motor and a power converter including: a converterunit for converting an alternating current voltage supplied from analternating current power supply into a direct current voltage, asmoothing capacitor for smoothing the converted direct current voltage,and an inverter unit for converting a direct current intermediatevoltage into an alternating current voltage having a frequencycorresponding to a torque instruction by a PWM control manner, bysetting an electromotive torque limit value and a regenerative torquelimit value in advance, and by restricting the torque instruction withthe torque limit values to generate a PWM switching pattern to be outputto the inverter unit, the control device comprising: a power failuredetecting unit which detects a power failure of the alternating currentpower source; a unit which outputs a deceleration start instruction tothe inverter unit in response to a power failure detecting signal outputfrom the power failure detecting unit, calculates a first reductionrate, on the basis of a detection value and a target value of the directcurrent intermediate voltage, such that the direct current intermediatevoltage is made constant during the deceleration of the alternatingcurrent motor, and calculates a second reduction rate on the basis of avariation in the direct current intermediate voltage; a unit whichcontrols a deceleration time by controlling a value obtained bymultiplying the two reduction rates together in a PI control manner; aunit which calculates the torque instruction to allow the alternatingcurrent motor to be decelerated for the deceleration time and changesthe electromotive torque limit value and the regenerative torque limitvalue on the basis of the value of the detected direct currentintermediate voltage; a unit which stops the deceleration when thedirect current intermediate voltage is equal to a voltage before thepower failure is detected or it rises during the deceleration; and aunit which stores an output frequency before the power failure isdetected when the alternating current motor is returned to a normalcontrol mode.
 5. The control device for an alternating current motoraccording to claim 4, wherein when the direct current intermediatevoltage is equal to the voltage before the power failure is detected orit rises during the deceleration, the deceleration stops, and when thealternating current motor is returned to the normal control mode, thealternating motor is accelerated for an arbitrary acceleration timeuntil its output frequency is equal to the output frequency storedbefore the power failure is detected.
 6. The control device for analternating current motor according to claim 4, wherein when the directcurrent intermediate voltage is equal to the voltage before the powerfailure is detected or it rises during the deceleration, thedeceleration stops, and when the alternating current motor is returnedto the normal control mode, the torque limit unit restricts the torqueinstruction with an arbitrary electromotive torque limit value until theoutput frequency of the alternating current motor is equal to the outputfrequency stored before the power failure is detected.
 7. A method ofcontrolling an alternating current motor and a power converterincluding: a converter unit for converting an alternating currentvoltage supplied from an alternating current power supply into a directcurrent voltage, a smoothing capacitor for smoothing the converteddirect current voltage, and an inverter unit for converting a directcurrent intermediate voltage into an alternating current voltage havinga frequency corresponding to a torque instruction by a PWM controlmanner, the method comprising the steps of: setting an electromotivetorque limit value and a regenerative torque limit value in advance;restricting the torque instruction with the torque limit values togenerate a PWM switching pattern to be output to the inverter unit;setting a lower limit voltage V_(U1) of the direct current intermediatevoltage required for a normal driving mode, a lower allowable voltageV_(U0) of the direct current intermediate voltage when a lowest voltageof a power supply is input, and a power failure detecting level voltageV_(U2) which is lower than the lower allowable voltage V_(U0) and ishigher than the lower limit voltage V_(U1); when a detection value ofthe direct current intermediate voltage output from a direct currentintermediate voltage detecting circuit is lower than the power failuredetecting level voltage V_(U2) during the driving of the alternatingcurrent motor, decelerating the alternating current motor at a reductionrate α_(d) until the direct current intermediate voltage is higher thanthe lower allowable voltage V_(U0); when the direct current intermediatevoltage is higher than the lower allowable voltage V_(U0), returning thealternating current motor to a normal control mode; and when the directcurrent intermediate voltage is higher than the lower allowable voltageV_(U0) to cause the alternating current motor to be returned to thenormal control mode, storing an output frequency before the powerfailure is detected.
 8. The method of controlling an alternating currentmotor according to claim 7, wherein when the direct current intermediatevoltage is higher than the lower allowable voltage V_(U0) to cause thealternating current motor to be returned to the normal control mode, thealternating current motor is accelerated for an arbitrary accelerationtime until its output frequency is equal to the output frequency storedbefore the power failure is detected.
 9. The method of controlling analternating current motor according to claim 7, wherein when the directcurrent intermediate voltage is higher than the lower allowable voltageV_(U0) to cause the alternating current motor to be returned to thenormal control mode, a torque limit unit restricts the torqueinstruction with an arbitrary electromotive torque limit value until theoutput frequency of the alternating current motor is equal to the outputfrequency stored before the power failure is detected.
 10. A controldevice which controls an alternating current motor and a power converterincluding: a converter unit for converting an alternating currentvoltage supplied from an alternating current power supply into a directcurrent voltage, a smoothing capacitor for smoothing the converteddirect current voltage, and an inverter unit for converting a directcurrent intermediate voltage into an alternating current voltage havinga frequency corresponding to a torque instruction by a PWM controlmanner, the control device controlling the alternating current motor inthe following sequence of: setting an electromotive torque limit valueand a regenerative torque limit value in advance; restricting the torqueinstruction with the torque limit values to generate a PWM switchingpattern to be output to the inverter unit; setting a lower limit voltageV_(U1) of the direct current intermediate voltage required for a normaldriving mode, a lower allowable voltage V_(U0) of the direct currentintermediate voltage when a lowest voltage of a power supply is input,and a power failure detecting level voltage V_(U2) which is lower thanthe lower allowable voltage V_(U0) and is higher than the lower limitvoltage V_(U1); when a detection value of the direct currentintermediate voltage output from a direct current intermediate voltagedetecting circuit is lower than the power failure detecting levelvoltage V_(U2) during the driving of the alternating current motor,decelerating the alternating current motor at a reduction rate α_(d)until the direct current intermediate voltage is higher than the lowerallowable voltage V_(U0); when the direct current intermediate voltageis higher than the lower allowable voltage V_(U0), returning thealternating current motor to a normal control mode; and when the directcurrent intermediate voltage is higher than the lower allowable voltageV_(U0) to cause the alternating current motor to be returned to thenormal control mode, storing an output frequency before the powerfailure is detected.
 11. The control device for an alternating currentmotor according to claim 10, wherein when the direct currentintermediate voltage is higher than the lower allowable voltage V_(U0)to cause the alternating current motor to be returned to the normalcontrol mode, the alternating current motor is accelerated for anarbitrary acceleration time until its output frequency is equal to theoutput frequency stored before the power failure is detected.
 12. Thecontrol device for an alternating current motor according to claim 10,wherein when the direct current intermediate voltage is higher than thelower allowable voltage V_(U0) to cause the alternating current motor tobe returned to the normal control mode, a torque limit unit restrictsthe torque instruction with an arbitrary electromotive torque limitvalue until the output frequency of the alternating current motor isequal to the output frequency stored before the power failure isdetected.
 13. A method of controlling an alternating current motor,comprising the steps of: providing a power failure detecting unit fordetecting a power failure of the alternating current motor in a powerconverter; outputting a deceleration start instruction to an inverterunit in response to a power failure detecting signal output from thepower failure detecting unit; calculating a first reduction rate, on thebasis of a detection value and a target value of a direct currentintermediate voltage, such that a direct current intermediate voltage ismade constant during the deceleration of the alternating current motor;calculating a second reduction rate on the basis of a variation in thedirect current intermediate voltage; controlling a deceleration time bycontrolling a value obtained by multiplying the two reduction ratestogether in a PI control manner; when the direct current intermediatevoltage is equal to a voltage before the power failure is detected or itrises during the deceleration, stopping the deceleration and returningthe alternating current motor to a normal control mode; and when thedirect current intermediate voltage is equal to the voltage before thepower failure is detected or it rises during the deceleration to causethe deceleration to stop and the alternating current motor to bereturned to the normal control mode, storing an output frequency beforethe power failure is detected.
 14. The method of controlling analternating current motor according to claim 13, wherein when the directcurrent intermediate voltage is equal to the voltage before the powerfailure is detected or it rises during the deceleration to cause thedeceleration to stop and the alternating current motor to be returned tothe normal control mode, the alternating current motor is acceleratedfor an arbitrary acceleration time until its output frequency is equalto the output frequency stored before the power failure is detected. 15.A control device which controls an alternating current motor,comprising: a power converter; and a power failure detecting unit whichdetects a power failure of the alternating current motor and is providedin the power converter, wherein a deceleration start instruction isoutput to an inverter unit in response to a power failure detectingsignal output from the power failure detecting unit, a first reductionrate is calculated, on the basis of a detection value and a target valueof a direct current intermediate voltage, such that a direct currentintermediate voltage is made constant during the deceleration of thealternating current motor, a second reduction rate is calculated on thebasis of a variation in the direct current intermediate voltage, a valueobtained by multiplying the two reduction rates together is controlledin a PI control manner to control a deceleration time, when the directcurrent intermediate voltage is equal to a voltage before the powerfailure is detected or it rises during the deceleration, a torqueinstruction is calculated to allow deceleration to stop and thealternating current motor to be decelerated for the deceleration time,and returning the alternating current motor to a normal control mode, anelectromotive torque limit value and a regenerative torque limit valueare changed on the basis of the value of the detected direct currentintermediate voltage, a torque limit unit restricts the torqueinstruction, and when the direct current intermediate voltage is equalto a voltage before the power failure is detected or it rises during thedeceleration to cause the deceleration to stop and the alternatingcurrent motor to be returned to the normal control mode, an outputfrequency before the power failure is detected is stored.
 16. The methodof controlling an alternating current motor according to claim 15,wherein when the direct current intermediate voltage is equal to thevoltage before the power failure is detected or it rises during thedeceleration to cause the deceleration to stop and the alternatingcurrent motor to be returned to the normal control mode, the alternatingcurrent motor is accelerated for an arbitrary acceleration time untilits output frequency is equal to the output frequency stored before thepower failure is detected.
 17. A method of controlling an alternatingcurrent motor, comprising the steps of: setting a lower limit voltageV_(U1) of a direct current intermediate voltage required for a normaldriving mode, a lower allowable voltage V_(U0) of the direct currentintermediate voltage when a lowest voltage of a power supply is input,and a power failure detecting level voltage V_(U2) which is lower thanthe lower allowable voltage V_(U0) and is higher than the lower limitvoltage V_(U1); when a detection value of the direct currentintermediate voltage output from a direct current intermediate voltagedetecting circuit is lower than the power failure detecting levelvoltage V_(U2) during the driving of the alternating current motor,decelerating the alternating current motor at a reduction rate α_(d)until the direct current intermediate voltage is higher than the lowerallowable voltage V_(U0); when the direct current intermediate voltageis higher than the lower allowable voltage V_(U0), returning thealternating current motor to a normal control mode; and when the directcurrent intermediate voltage is higher than the lower allowable voltageV_(U0) to cause the alternating current motor to be returned to thenormal control mode, storing an output frequency before the powerfailure is detected.
 18. The method of controlling an alternatingcurrent motor according to claim 17, wherein when the direct currentintermediate voltage is higher than the lower allowable voltage V_(U0)to cause the alternating current motor to be returned to the normalcontrol mode, the alternating current motor is accelerated for anarbitrary acceleration time until its output frequency is equal to theoutput frequency stored before the power failure is detected.
 19. Acontrol device for an alternating current motor, comprising: a sequenceunit which performs the functions of: setting a lower limit voltageV_(U1) of a direct current intermediate voltage required for a normaldriving mode, a lower allowable voltage V_(U0) of the direct currentintermediate voltage when a lowest voltage of a power supply is input,and a power failure detecting level voltage V_(U2) which is lower thanthe lower allowable voltage V_(U0) and is higher than the lower limitvoltage V_(U1); when a detection value of the direct currentintermediate voltage output from a direct current intermediate voltagedetecting circuit is lower than the power failure detecting levelvoltage V_(U2) during the driving of the alternating current motor,decelerating the alternating current motor at a reduction rate α_(d)until the direct current intermediate voltage is higher than the lowerallowable voltage V_(U0); and when the direct current intermediatevoltage is higher than the lower allowable voltage V_(U0), returning thealternating current motor to a normal control mode, when the directcurrent intermediate voltage is higher than the lower allowable voltageV_(U0) to cause the alternating current motor to be returned to thenormal control mode, storing an output frequency before the powerfailure is detected.
 20. The control device for an alternating currentmotor according to claim 19, wherein when the direct currentintermediate voltage is higher than the lower allowable voltage V_(U0)to cause the alternating current motor to be returned to the normalcontrol mode, the alternating current motor is accelerated for anarbitrary acceleration time until its output frequency is equal to theoutput frequency stored before the power failure is detected.